Catalytic combustion of heavy oil in presence of manganese-based

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Catalytic combustion of heavy oil in presence of manganesebased sub-micro particles in a quartz porous medium Andrey V. Galukhin, Mohammed Amine Khelkhal, Alexey V. Eskin, and Yuri N. Osin Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b01856 • Publication Date (Web): 30 Aug 2017 Downloaded from http://pubs.acs.org on August 31, 2017

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Catalytic combustion of heavy oil in presence of manganese-based sub-micro particles in a quartz porous medium Andrey V. Galukhin,* Mohammed A. Khelkhal, Alexey V. Eskin, Yuri N. Osin Kazan Federal University, 18 Kremlevskaya str., Kazan 420008, Russian Federation

KEYWORDS In situ combustion; catalytic combustion; manganese oxide; sub-micro particles; thermal analysis; isoconversional approach.

ABSTRACT. We synthesized manganese oxide (II) sub-micro particles stabilized with oleic acid and used them as oil dispersible catalyst precursor promoting heavy oil oxidation process. Size, morphological properties, as well as composition of the catalyst were profoundly studied by Xray phase diffraction analysis, TG-MS analysis, SEM, energy dispersive X-ray analysis, and nitrogen adsorption measurements. We applied non-isothermal kinetic analysis coupled with isoconversional approach to study influence of catalyst on combustion process, and showed that Mn-based sub-micro particles accelerate predominantly high temperature oxidation process.

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1. Introduction Smoldering combustion of organic matter dispersed in porous media is widely spread phenomenon in nature. For instance, smoldering of the forest ground, peatlands and coal mines leads to terrible implications and may causes significant environmental problems.1 Besides aforementioned negative cases, smoldering combustion in oil reservoirs, well known as in situ combustion (ISC), is a promising approach to development of oilfields. During ISC, air is injected into a reservoir to oxidize a small portion of the hydrocarbons and generates heat and pressure that enhance oil recovery. Stability of the smoldering front is considered to be a key parameter determining success of the ISC process.2 One of the possible ways to increase stability of the combustion front is application of catalysts, which can promote oxidation process and improve oil properties.3 Manganese based catalysts stand out of the row of the catalysts made of other transition metals due to their high catalytic activity for both full4,5 and partial6,7 oxidation of organic matter, low toxicity and availability. This set of properties makes Mn-based catalysts excellent candidate to facilitate smoldering combustion of organic matter. In our previous study8 we showed that catalytic species formed from manganese (III) acetylacetonate precursor significantly accelerate combustion of heavy oil. We proposed an approach based on combination of X-ray phase diffraction analysis (XRPD), thermogravimetry and electron paramagnetic resonance spectroscopy (EPR) to reveal that catalytic cycle includes transitions of manganese between II and III oxidation states. We also showed that initial organic chelate complex transform into inorganic manganese oxide particles which further participate in oxidation process.


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Current study is a next logical step in our systematic investigation of catalytic oil combustion. We prepared oleophilic manganese (II) oxide sub-micro particles and studied their influence on heavy oil combustion. The choice of sub-micro sized particles as a form of a catalyst was based on a simplicity of the synthesis and isolation, as well as its higher aggregation stability in solutions compared with nanoparticles. 2. Experimental Section 2.1. Materials The crude oil used in this research was extracted from oil core taken from the Ashalcha oilfield (Volga-Ural basin, Republic of Tatarstan, Russia).9,10 The physical properties, elemental and SARA (Saturated and Aromatic hydrocarbons, Resins and Asphaltenes) analysis data of crude oil are given in Table 1. All organic solvents were purchased from local company and used without additional purification (purity was more than 99.5%). Oleic acid (technical grade, 90%), manganese (II) nitrate tetrahydrate (99%) was purchased from Sigma Aldrich. Quartz sand was purchased from PJSC "Gusevskiy Glassworks named after Dzerzhinsky". Table 1. Physical properties of Ashalcha heavy oil at 20 °С. Elemental content, %

SARA analysis, %

Viscosity,

Density,

API

mPa×s

g×cc-1

gravity

C

H

N

S

Saturated

Aromatic

Resins

Asphaltenes

11 811

0.97

13.8

82.09

10.12

0.63

2.65

26.2±0.5

44.1±0.6

26.3±0.5

4.5±0.3

2.1.1 MnO sub-micro particles preparation Initial manganese (II) oleate was synthesized according to well-known procedure.11 13.87 g (22.4 mmol) of synthesized manganese (II) oleate was mixed with 3.50 g (12.4 mmol) of oleic


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acid and 180 ml of heptane in a Parr Instrument 4560 autoclave. The resulting mixture was flushed with nitrogen and then heated up to 300 °C at an heating rate of 5 °C/min with constant stirring. Then the mixture was hold at that temperature for 30 min, then slowly cool down to room temperature. The particles were isolated from reaction mixture by centrifugation, washed several times with heptane and methanol and dried in nitrogen flow. 2.1.2 Samples preparation for kinetic analysis Samples for TG analysis were prepared by mixing the heavy oil (10.0 wt %) with pure quartz sand ((fraction of 43-64 µm), 90.0 wt %). In case of catalytic experiments an initial oil sample contained 2.0 wt % of sub-micro particles.

2.2 Methods 2.2.1 Thermal analysis TGA and DSC experiments were carried out with thermal analyzer Netzsch STA 449 F3 Jupiter. The experiments were conducted at linear heating rates of 5, 10, 15, 20 °С/min for kinetic analysis of heavy oil samples, under argon flow (75 ml/min). The samples were heated from ambient temperature up to 600°С. An average sample was 10 mg for each run. 2.2.2 Scanning electron microscopy and energy dispersive X-ray analysis Scanning electron microscopy (SEM) measurements and energy dispersive X-ray analysis (EDX) were carried out by using field-emission high-resolution scanning electron microscope Merlin Carl Zeiss equipped with Energy Dispersive Spectrometer AZtec X-MAX. Observation


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photos of morphology surface were obtained at accelerating voltage of incident electron 15 kV and current probe 300 pA. During the elemental analysis of the surface accelerating voltage of incident electron was 5 kV, working distance 9 mm. Probing depth was about 1 µm. 2.2.3 X-ray powder diffraction X-ray powder diffraction (XRPD) studies were made using a MiniFlex 600 diffractometer (Rigaku) equipped with a D/teX Ultra detector. Cu Kα1 radiation (40 kV, 15 mA) was used and data were collected at 25°C in the range of 2θ from 2° to 100° with a step of 0.02° and exposure time at each point of 0.24 s without sample rotation. 2.2.4 Specific surface area measurements Specific surface area (SSA) of the catalyst was measured by nitrogen adsorption at 77 K with ASAP 2020 MP (Micromeritics) instrument. About 0.5 g of sample was degassed by heating at 200 °C under low pressure of 8 µmHg during 2 hours. SSA value was determined by applying the Brunauer−Emmett−Teller (BET) equation.

3. Results and discussion It is widely known that phase content and morphological properties of the heterogeneous catalysts influence on their performance. Therefore primarily we profoundly studied characteristics of the synthesized catalyst. It turns out, applied synthetic procedure allows to obtain highly crystalline manganese oxide (II) particles without any impurities of other manganese oxides that proved by XRPD analysis (Figure 1).


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Figure 1. XRPD analysis of synthesized MnO sub-micro particles.

To examine size and morphology of obtained particles we applied scanning electron microscopy (SEM), that revealed (Figure 2) microcrystals possess a form of octahedron (Figure 2 inset) with average size of 120±34 nm (random edge length calculated from 100 measurements). Energy dispersive X-Ray analysis (EDX) of microparticles’ surface showed presence of carbon from oleic acid (Figure 3).

Figure 2. SEM image of synthesized MnO sub-micro particles.


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Figure 3. EDX spectrum of synthesized MnO sub-micro particles.

We also used TG analysis to evaluate content of the oleic acid on the surface of the obtained sub-micro particles. Total mass loss during heating up procedure was 3.3 %. As we can see (Figure 4) at first step insignificant mass loss related to removal of physically adsorbed water (about 0.3 %) takes place. At second step we observe simultaneous decarboxylation of oleic acid and removal of chemically adsorbed water. Total decomposition of organic matter occurs at third step. As we can see total decomposition of the organic part of the catalyst in argon atmosphere ends up at a temperature of about 450 °C, where we can consider the surface of our catalyst free of organic surfactant. In air atmosphere this process likely proceeds faster and ends up at lower temperature.


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Figure 4. TG, DTG and ion current curves for synthesized MnO sub-micro particles (argon atmosphere, heating rate of 10 K×min-1). Specific surface area (SSA) is a very important characteristic determining activity of dispersed catalysts; therefore, we conducted nitrogen adsorption measurements to evaluate SSA of synthesized sub-micro particles. Figure 5 shows nitrogen adsorption and desorption isotherms, obtained SSA value was 12.85±0.19 m2×g-1.

Figure 5. Nitrogen adsorption−desorption isotherms at 77 K.

The theoretical value of SSA calculated with assumption that sub-micro particles have a form of right octahedron with an edge size of 120 nm (Figure 2), yields in SSA value of 11.28 m2×g (density of the MnO is assumed to be 5.43 g×cm-3). Closeness of theoretical and measured SSA


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evidences about nonporous nature of the catalyst, which is in good agreement with the results obtained by SEM. Based on known catalyst’s SSA value (12.85 m2×g-1), content of oleic acid (3%) and its molecular mass (282.5 g×mol-1) one can calculate the amount of surfactant molecules per 1 nm2 of the catalyst surface, which is 5.1 molecules×nm-2. The cross sectional area of oleic acid12 is 0.46 nm2 means that average amount of oleic acid on a surface cannot exceed 2.2 molecules×nm-2 in case of monolayer coverage. Thus we guess that surfactant covers the surface of obtained MnO particles by 2-3 layers.

3.2. Combustion study by DSC and kinetic analysis DSC curves related to non-catalytic and catalytic combustion of heavy crude oil performed at different heating rates are given on Figure 6. During combustion two main oxidation regions, namely low temperature oxidation (LTO) and high temperature oxidation (HTO), are clearly seen on DSC curves. LTO yields in partially oxygenated hydrocarbons (peroxides, alcohols, carbonyl compounds) without producing significant amount of carbon oxides.13 HTO is usually described as coke oxidation producing carbon oxides and water.14


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Figure 6. DSC curves for non-catalytic (top) and catalytic (bottom) heavy oil combustion.

DSC reaction intervals of both regions for non-catalytic and catalytic processes as well as peak temperatures are given in Table 2. Table 2. DSC reaction intervals and peak temperatures. Non-catalytic Sample β, °С×min

Catalytic

-1

Interval, °C Tp, °C Interval, °C Tp, °C 5

248-380

343

267-364

340

10

276-409

364

257-383

358

15

280-422

376

282-397

369

20

291-426

386

297-406

377

5

433-489

473

422-457

441

10

439-514

494

436-477

462

15

444-528

502

446-493

474

20

450-538

512

458-503

483

LTO

HTO

It can clearly be seen peak temperatures of LTO regions are quite close to each other for corresponding heating rates for non-catalytic and catalytic processes, whereas peak temperatures


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of catalytic HTO are significantly lower. Figure 7 shows the differences between corresponding pairs of Tp values for low and high temperature oxidation.

Figure 7. Differences between the peak temperatures (∆Tp) for catalytic and non-catalytic heavy oil oxidation at different heating rates.

Oil oxidation can be described as a simple reaction rate model that assumes functional dependency on oil conversion degree α, and oxygen partial pressure.13 This widely accepted model is given by following Equation 1: 1 1 The rate constant k(T) is assumed to obey the Arrhenius law (Equation 2): / 2 The conversion degree α in DSC is calculated according Equations 3:

/

3 /

Early study conducted by Bousaid and Ramey shows a first-order dependency with respect to both oil concentration and oxygen partial pressure.15 Since the small amount of oil used for each


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DSC run in our study (about 1 mg) and high air flow rate of 75 ml/min combined with large furnace volume (approx. 250 ml), we assume that oxygen partial pressure remains constant (21.2 kPa) during the experiment. Considering all the above we express the final reaction model as described in Equation 4. 1 4,

where

We used Kissinger’s method16 to study kinetics of the heavy oil combustion. The method links variation of peak temperature from heating rate with kinetic parameters of the process, logarithmic form of the equation is presented by Equation 5. '(

) , 1 . . / '( 5 + * - * ,

Kissinger’s method was chosen over various of other integral and differential methods due to strong dependence of DSC data from the baseline choice. Since Tp values are almost independent of baseline choice the Kissinger’s method was the best alternative. Figure 8 shows Kissinger’s plots for low and high temperature oxidation in presence and absence of the catalyst. The kinetic parameters are grouped in Table 3.


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Figure 8. Kissinger’s plots for catalytic and non-catalytic high temperature oxidation (top is for LTO, bottom is for HTO).

Table 3. Kinetic parameters of combustion processes. Non-catalytic LTO

HTO

Catalytic LTO

HTO

Ea, kJ/mol 100.0±1.8 161.7±8.3 115.0±0.7 137.1±0.1 lgA

4.6

7.6

6.1

6.2

It should be noted calculated A and Ea parameters make opposite contributions into reaction rate constant k(T) for both LTO and HTO processes. In case of LTO the catalytic process possesses higher Ea (which decreases k(T)) and higher preexponential factor A (which increases k(T)) compared with non-catalytic reaction. Similar situation occurred for HTO: catalytic process possesses lower Ea (increasing k(T)) and lower A (decreasing k(T)) compared with non-catalytic reaction. To estimate combined effect of these two parameters we calculated k(T) values for LTO and HTO over broad temperature ranges where these processes occurred (Table 2).


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Comparison of effective rate constants for non-catalytic and catalytic oxidation process is presented on Figure 9.

Figure 9. Variation of effective rate constants with temperature for catalytic and non-catalytic oxidation (top is for LTO, bottom is for HTO).

As we can see from Figure 9 non-catalytic and catalytic combustion show minimal differences for LTO, whereas in case of HTO, catalytic combustion rate is significantly higher. Thus, we can conclude that proposed Mn-based sub-micro particles effectively catalyze heavy crude oil oxidation process and might be applied for stabilization of combustion front. Based on the results published in our previous study8 we propose MnO sub-micro particles in the experiment’s conditions transform into Mn3O4 which is supposed to be the real catalyst.


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4. Conclusions We synthesized manganese oxide (II) sub-micro particles stabilized with oleic acid and profoundly studied their composition, size and morphological properties. We applied nonisothermal kinetic analysis coupled with isoconversional approach to study influence of catalyst on combustion process, and showed that synthesized Mn-based sub-micro particles accelerate predominantly high temperature oxidation process. Further improvement of catalytic performance may be reached by adjustment of size and morphology of the particles to increase surface area available for reaction, which is a theme of our future study.

AUTHOR INFORMATION Corresponding Author *Corresponding author. E-mail address: [email protected] Funding Sources The work was supported by grant №14.Y26.31.0019 from Ministry of Education and Science of Russian Federation.

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Catalytic combustion of heavy oil in presence of manganese-based

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